The invention relates to methodology and apparatus to determine the location and orientation of an object, for example a medical device, located inside or outside a body, while the body is being scanned by magnetic resonance imaging (MRI). More specifically, the invention enables estimation of the location and orientation of various devices(e.g. catheters, surgery instruments, biopsy needles, etc.) by measuring voltages induced by time-variable magnetic fields in a set of miniature coils. Such time-variable magnetic fields are generated by an MRI scanner during its normal operation.
Minimally invasive procedures: Minimally-invasive diagnostic or interventional procedures require either direct visual viewing or indirect imaging of the field of operation and determination of the location and orientation of the operational device. For example, laparoscopic interventions are controlled by direct viewing of the operational field with rigid endoscopes, while flexible endoscopes are commonly used for diagnostic and interventional procedures within the gastrointestinal tract. Vascular catheters are manipulated and manoeuvred by the operator, with real-time X-ray imaging to present the catheter location and orientation. Ultrasound imaging and new real-time MRI and CT scanners are used to guide diagnostic procedures (e.g. aspiration and biopsy) and therapeutic interventions (e.g. ablation, local drug delivery) with deep targets. While the previous examples provide either direct (optical) or indirect (imaging) view of the operation field and the device, another approach is based on remote sensing of the device with mechanical, optical or electromagnetic means to determine the location and orientation of the device inside the body.
Stereotaxis: Computer-assisted sterotaxis is a valuable technique for performing diagnostic and interventional procedures, most typically with the brain. The concept behind the technique is to have real-time measurement of the device location in the same coordinate system as an image of the field of operation. The current location of the device and its future path are presented in real-time on the image and provide the operator with feed-back to manipulate the device with minimal damage to the organs. During traditional sterotaxis, the patient wears a special halo-like headframe, which provides the common coordinate system, and CT or MRI scans are performed to create a three-dimensional computer image that provides the exact location of the target (e.g. tumour) in relation to the headframe. The device is mechanically attached to the frame and sensors provide its location in relation to the head frame. When this technique is used for biopsy or minimally-invasive surgery of the brain, it guides the surgeon in determining where to make a small hole in the skull to reach the target. Newer technology is the frameless technique, using a navigational wand without the headframe (e.g. Nitin Patel and David Sandeman, xe2x80x9cA Simple Trajectory Guidance Device that Assists Freehand and Interactive Image Guided Biopsy of Small Deep Intracranial Targetsxe2x80x9d, Comp Aid Surg 2:186-192, 1997). In this technique remote sensing system (e.g. light sources and sensors) provides the real-time location of the device with respect to the image coordinate system. Yet both the sterotaxis and the frameless techniques are typically limited to the use of rigid devices like needles or biopsy forceps since their adequate operation requires either mechanical attachments or line of sight between the light sources and the sensors.
Electromagnetic remote sensing: Newer remote sensing techniques are based on electromagnetism. For example, Acker et al (U.S. Pat. No. 5,558,091) disclose such a method and apparatus to determine the position and orientation of a device inside the body. This method uses magnetic fields generated by Helmholtz coils, and a set of orthogonal sensors to measure components of these fields and to determine the position and orientation from these measurements. The measurement of the magnetic field components is based on Hall effect and requires exciting currents in the sensors in order to generate the measured signals. The technique requires control of the external magnetic fields and either steady-state or oscillating fields, for the induced voltages to reach a state of equilibrium. These requirements prevent, or greatly complicate, the use of this technique with magnetic fields generated by the MRI system, and requires the addition of a dedicated set of coils to generate the required magnetic fields.
A different approach for remote sensing of location is disclosed by Pfeiler et al. (U.S. Pat. No. 5,042,486) and is further used by Ben-Haim for intra-body mapping (U.S. Pat. No. 5,391,199). Their technology is based on generating weak radio-frequency (RF) signals from three different transmitters, receiving the signals through an RF antenna inside the device, and calculating the distances from the transmitters, which define the spatial location of the device. As with the previous methodology, the application of the technology to MRI is problematic due to the simultaneous use of RF signals by the MR scanning. Potential difficulties are the heating of the receiving antenna in the device by the high amplitude excitation RF transmissions of the MRI scanner and artifacts in the MR image.
Dumoulin and colleagues disclose another approach to determine the location of a device, using a small receiving coil which is sensitive to near-neighbourhood emitted RF signal during the MR imaging process (Dumoulin C L, Darro R D, Souza S P, xe2x80x9cMagnetic resonance trackingxe2x80x9d, in Interventional MR, edited by Jolesz F A and Young I Y, Mosby, 1998). This method cannot directly determine the orientation of the device, and may be subject to similar difficulties as the previous technology, including heating of the coil.
Interventional MRI: Many of the advantages of MRI that make it a powerful clinical imaging tool are also valuable during interventional procedures. The lack of ionizing radiation and the oblique and multi-planar imaging capabilities are particularly useful during invasive procedures. The absence of beam-hardening artifacts from bone allows complex approaches to anatomic regions that may be difficult or impossible with other imaging techniques such as conventional CT. Perhaps the greatest advantage of MRI is the superior soft-tissue contrast resolution, which allows early and sensitive detection of tissue changes during interventional procedures. Many experts now consider MRI to be one of the most powerful imaging techniques to guide interventional interstitial procedures, and in some cases even endovascular or endoluminal procedures (Yoshimi Anzai, Rex Hamilton, Shantanu Sinha, Antonio DeSalles, Keith Black, Robert Lufkin, xe2x80x9cInterventional MRI for Head and Neck Cancer and Other Applicationsxe2x80x9d, Advances in Oncology, May 1995, Vol 11 No. 2).
From the present background on current methodologies, one can define the ideal system for minimal invasive procedures: It should provide real-time, 3-dimensional, non-ionizing imaging (like MRI or ultrasound) as feed-back to the user for optimal insertion and intervention; it should implement flexible, miniaturized devices which are remotely sensed to provide their location and orientation. By combining a composite image of the field of operation and the device location and orientation, the operator can navigate and manipulate the device without direct vision of the field of operation and the device. This may facilitate the use of minimal invasive intervention in the brain or other organs.
An object of the present invention is to provide a novel method and apparatus for determining the instantaneous location and orientation of an object moving through a three-dimensional space, which method and apparatus have advantages in one or more of the above respects.
Another object of the present invention is to provide such a method and apparatus which is particularly useful in MRI systems by making use of a basic universal component of the MRI system, namely the time-varying magnetic gradients which are typically generated by a set of three orthogonal electromagnetic coils in such systems.
According to one aspect of the present invention, there is provided a method of determining the instantaneous location and orientation of an object moving through a three-dimensional space, comprising: applying to the object a coil assembly including a plurality of sensor coils having axes of known orientation with respect to each other and including components in the three orthogonal planes; generating a time-varying, three-dimensional magnetic field gradient having known instantaneous values of magnitude and direction; applying the magnetic field gradient to the space and the object moving therethrough to induce electrical potentials in the sensor coils; measuring the instantaneous values of the induced electrical potentials generated in the sensor coils; and processing the measured instantaneous values generated in the sensor coils, together with the known magnitude and direction of the generated magnetic field gradient and the known relative orientation of the sensor coils in the coil assembly, to compute the instantaneous location and orientation of the object within the space.
The above-described method is particularly useful in MRI systems, wherein the magnetic field gradient is generated by activating the gradient coils of an MRI scanner, and the invention is therefore described below with respect to such a system.
According to further features in the described preferred embodiment, the magnetic field gradient is generated by activating three orthogonally disposed pairs of gradient coils according to a predetermined activating pattern; and the measured instantaneous values of the induced electrical potentials generated in the sensor coils are processed, together with the predetermined activating pattern of the gradient coils and the known relative orientation of the sensor coils, to provide an estimate of the location and orientation of the object.
According to another aspect of the present invention, there is provided apparatus for determining the instantaneous location and orientation of an object moving through a three-dimensional space, comprising: a coil assembly carried by the object and including a plurality of sensor coils having axes of known orientation with respect to each other and including components in the three orthogonal planes; a magnetic field generator generating for a time-varying, three-dimensional magnetic field gradient having known instantaneous values of magnitude and direction in the space and the object moving therethrough to induce electrical potentials in the sensor coils; means for measuring the instantaneous values of the induced electrical potentials generated in the sensor coils; and a processor for processing the measured instantaneous values generated in the sensor coils, together with the known magnitude and direction of the generated magnetic field gradient and the known relative orientation of the sensor coils in the coil assembly, to compute the instantaneous location and orientation of said object within said space.
The disclosed methodology and apparatus enable the estimation of the location and orientation of an object or a device by using a set of miniature, preferably (but not necessarily) orthogonal coils. The simplest, preferred embodiment has a set of three orthogonal coils. However more complex coil sets, for example a set of three orthogonal pairs of parallel coils, can improve the accuracy of the tracking with a higher cost of the system. To simplify the presentation, the following disclosure deals specifically with a set of three orthogonal coils, and also refers to the more complex configuration of three orthogonal pairs of coils. However the same concepts can be applied to various combinations of coils by anyone familiar with the field of the invention.
The time change of magnetic flux through a coil induces electromotive force (i.e. electric potential) across the coil (Faraday Law of electromagnetism). MRI scanners generate time-variable magnetic fields to create magnetic gradients in the scanned volume. By measuring the induced electric potentials in the three orthogonal coils (or pairs of coils), and by getting the time pattern of the generated magnetic gradients as input from the MRI scanner, both the location and orientation of the device can be estimated.
The present invention has significant advantages over existing methodologies. Compared with sterotaxis, either the frame or frameless techniques, the new methodology enables the use of devices like catheters or surgical instrumentation without the need for direct line of sight with the device. Unlike the remote electromagnetic localization methodology of Acker et al the present invention is based on measurement of voltages induced by a set of time-varying electromagnetic gradient fields in a set of coils (Faraday Law), rather than the need to use homogenous and gradient fields which induce voltages in a set of miniature conductors carrying electrical current (Hall effect). Thus, the present invention is totally passive, it does not require any excitation of the sensors, nor the use of dedicated magnetic fields, and the requirement for time-variable magnetic fields is satisfied with virtually any MRI scanning protocol which is in routing clinical use. The methods disclosed by Pfeiler et al and Dumouline et al require the use of two sensors to measure orientations and thus have limited accuracy of orientation estimation, while the present invention uses a sensor which provides simultaneously accurate orientation and location tracking. Unlike existing optical tracking systems, there is no limitation on the number of sensors being used, and there is no need to maintain a line of sight between the sensor and the tracking apparatus. All other tracking methodologies are based on their own reference system, and should be aligned with the MRI coordinate system by a time-consuming registration procedure. The disclosed tracking methodology does not require registration since it uses the same set of gradient coils which are used by the MRI scanner for spatial encoding of the images.